The reaction to be transmitted from the steered road wheels to the steering wheel of an automotive vehicle varies with the vehicle speed, generally decreasing as the vehicle speed increases and increasing as the vehicle speed decreases. In an automotive vehicle equipped with a power-assisted steering system in which fluid under pressure is used to boost the manual steering effort applied to the steering wheel, a considerably large amount of energy is thus consumed for driving the steering pressure pump especially at low vehicle speeds. If the steering pressure pump is driven by the internal combustion engine of the vehicle, the power output of the engine is thus consumed in notable proportion for the driving of the steering pressure pump and will cause obstruction to improving the fuel economy of the engine.
In an effort to solve such a problem, a power-assisted steering system has been proposed in which the steering pressure pump is put into operation to develop a steering assistance fluid pressure only when the vehicle is being steered and in which the hydraulic steering assistance is controlled to vary with vehicle speed. A typical example of such a power-assisted steering system is disclosed in Japanese Provisional Patent Publication No. 56-99859. Problems have however been still encountered in such a power-assisted steering system as will be discussed later. The present invention contemplates resolution of such problems.